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Web presentation layer patterns

The Model-View-Controller (MVC) architectural pattern

Catalog of MVC-based patterns

Other patterns

Web presentation-business integration patterns

Business layer architectural patterns

MVC Web frameworks

Unit 7: Design Patterns and Frameworks

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Divides an interactive application into three components/levels:

Model:

Contains the functional core of the application

Encapsulates the appropriate data, and exports procedures that perform application-specific processing

View:

Displays information to the user

Obtains the data from the model

Different views present the model’s information in different ways

Controller:

Accepts user input as events

Translates events to service requests for the model or display requests for the view

The MVC Architectural Pattern

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The “Classical” MVC Pattern

Controller

initialize( m : Model, v : View )

handleEvent()

update()

Model

data

attach( ob : Observer )

detach( ob : Observer )

notify()

getData()

service()

View

initialize( m : Model )

makeController()

activate()

display()

update()

Observer

update()

*

1

11

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Controller Patterns:

Page Controller

Front Controller

Application Controller

Intercepting Filter

View Patterns

View Helper

Composite View

Transform View

Composite Patterns:

Dispatcher View

Service to Worker

Catalog of MVC-Based Web Presentation Patterns

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Page Controller

Each Page Controller component act as the controller for a dynamic page on the Web site.

The basic responsibilities of a Page Controller are:

Decode the URL and extract any form data

Invoke model components to process the request data

Determine which view should display the result page and forward the model information to it

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Page Controller: How It Works

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Page Controller works particularly well in a site where most of the controller logic is pretty simple:

Variant: Page Controller and View implemented by the same Server Page: (Model 1)

Page Controller: When to Use It

: User : System

service(parameters)

result

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Problem: The system requires a centralized access point for request handling. Without a central access point, control code that is common across multiple requests is duplicated in numerous places. When control code is intermingled with view-creation code, the application is less modular and cohesive.

Forces:

You want to avoid duplicate control logic.

You want to apply common logic to multiple requests.

You want to separate system processing logic from the view.

You want to centralize controlled access points into your system

Solution: Use a Front Controller as the initial point of contact for handling all related requests. The Front Controller centralizes control logic that might otherwise be duplicated, and manages the key request handling activities .

Front Controller (Model 2)

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Front Controller: Class Diagram

1

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Front Controller: Sequence Diagram

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Front Controller + Application Controller

An Application Controller has two main responsibilities: deciding which domain logic to run and deciding the view with which display the response.

It is useful for designing complex use cases with definite rules about the order in which pages should be visited and different views depending on the state of objects.

Separating the Application Controller from the Front Controller allows improving the modularity of the system, while enhancing its reusability.

The Application Controller component is not a Server Page

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Front Controller + Application Controller (cont.)

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Application Controller with Mapper

Application Controller: Uses Mapper to resolve an incoming request to the appropriate action and view, to which it delegates or dispatches.

Mapper: Uses a Map to translate an incoming request into the appropriate action and view. A Mapper acts as a factory

Map: Acts as a dictionary or registry of references to target resources.

Target: A resource that helps fulfill a particular request (view, command)

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Application Controller with Mapper (cont.)

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Application Controller with Mapper (cont.)

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Problem: You want to intercept and manipulate a request and a response before and after the request is processed in order to determine if:

The client has a valid session

The request path violates any constraint

You support the browser type of the client

The client has used a special encoding to send the data

The request stream is encrypted or compressed

Forces

You want centralized, common processing across requests

You want pre and postprocessing components loosely coupled with core request-handling services.

You want pre and postprocessing components independent of each other and self contained to facilitate reuse.

Solution: Use an Intercepting Filter as a pluggable filter to pre and postprocess requests and responses. A Filter Manager combines loosely coupled filters in a chain, delegating control to the appropriate filter.

Intercepting Filter

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Intercepting Filter: Structure, Participants and

Responsibilities

FilterManager: Manages filter processing. It creates the FilterChain with the appropriate filters, in the correct order, and initiates processing.

FilterChain: Ordered collection of independent filters

Filter: Component that performs a specific pre and/or postprocessing task.

Target: The resource requested by the client.

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Intercepting Filter: Sequence Diagram

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Motivation: HTML forms that include a file upload use a different encoding type than that of basic forms. As a result, form data that accompanies the upload is not available via simple getParameter() invocations. The following filter preprocesses requests to translate the encoding type into a single consistent format that makes all form data available as request attributes:

public class MultipartEncodeFilter extends javax.servlet.Filter { public void doFilter(javax.servlet.ServletRequest request,

javax.servlet.ServletResponse response,

javax.servlet.FilterChain filterChain)

throws java.io.IOException, javax.servlet.ServletException {

String contentType = request.getContentType();

if (contentType.startsWith("multipart/form-data") ) {

For each pair (attributeName, value) obtained from the request do request.setAttribute(attributeName, value)

}

filterChain.doFilter(request, response); } }

Java Filters: Example

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Problem: You want to separate a view from its processing logic. JSP and other template-based views allow embedding scriptlet code

Embedded scriptlet code cannot be reused easily

Embedded scriptlet code often acts as control code or performs view preparation activities, such as content retrieval.

Mingling control logic, data access logic and formatting logic leads to problems with modularity, reuse, maintenance, and role separation.

Forces: You want to use template-based views, such as JSP.

You want to avoid embedding program logic in the view.

You want to separate programming logic from the view to facilitate division of labor between software developers and web page designers

Solution: Use Views to encapsulate formatting code and Helpers to encapsulate view-processing logic. A View delegates its processing responsibilities to its helper classes,

implemented as POJOs, custom tags, or tag files.

Helpers serve as adapters between the view and the model, and perform processing related to formatting logic, such as generating an HTML table.

View Helper

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View Helper: Structure

Helper: Encapsulates processing logic for generating and formatting a View. A helper typically adapts a PresentationModel for a view or provides access to the raw data of the PresentationModel

PresentationModel: Holds the data retrieved from the business service, used to generate the View. It can be either a Business Delegate or a DTO

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View Helper: Sequence Diagram

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<%@ taglib uri=http://java.sun.com/jsp/jstl/core

prefix="c" %>

<jsp:useBean id="welcomeHelper" scope="request"

class="WelcomeHelper" />

<HTML><BODY bgcolor="FFFFFF">

<c:if test = "${welcomeHelper.nameExists == true}">

<center><H3> Welcome <b>

<c:out value='${welcomeHelper.name}'/></b><br><br></H3>

</center>

</c:if>

<H4><center>We are happy for your visit!</center></H4>

</BODY>

</HTML>

View Helper: Example with JavaBean and JSTL

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Problem: You want to build a view from modular, atomic component parts that are combined to create a composite whole, while managing the content and the layout independently.

Forces You want common subviews, such as headers, footers and

tables reused in multiple views, which may appear in different locations within each page layout.

You have content in subviews that frequently changes or might be subject to certain access controls, such as limiting access to users in certain roles.

You want to avoid directly embedding and duplicating subviews in multiple views which makes layout changes difficult to manage and maintain.

Solution: Use Composite Views that are composed of multiple atomic subviews. Each subview of the overall template can be included dynamically in the whole, and the layout of the page can be managed independently of the content.

Composite View

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Composite View: Structure

Template: Represents the page layout

ViewManager: Uses a Template to enforce a layout into which it places the appropriate content.

A simple ViewManager might use the standard JSP include tag (<jsp:include>) to include SimpleView segments into a Template.

A more sophisticated ViewManager might use POJOs or custom tag helpers to provide content and layout management in a more comprehensive and robust manner.

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Composite View: Sequence Diagram

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Problem: You want a view that processes domain data element by element and transforms it into HTML.

Forces: The data returned by the business layer is either in XML or in something automatically transformable to it

Solution: Use a Transform View that transforms directly from domain-oriented XML into (X)HTML by using XSLT (eXtensible Stylesheet LanguageTransformations). Normally XSLT does this by transforming each XML element into an (X)HTML element.

Transform View

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Problem: You want a view to handle a request and generate a response, with little or no business processing performed before rendering the view.

Forces:

You have static views

You have views generated from an existing presentation model

You have views which are independent of any business service response

You have limited business processing

Solution: Use Dispatcher View with views as the initial access point for a request. Business processing, if necessary in limited form, is managed by the views.

Dispatcher View

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Dispatcher View: Structure

BusinessHelper: Helps the View initiate business processing to handle a request

BusinessService: Encapsulates the business logic and business state. A remote business service is accessed via a Business Delegate

PresentationModel: Holds the data retrieved from the business service (DTO).

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Dispatcher View: Sequence Diagram

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Problem: You want to perform core request handling and invoke business logic before control is passed to the view

Forces:

You want specific business logic executed to service a request in order to retrieve content that will be used to generate a dynamic response.

You have view selections which may depend on responses from business service invocations.

You may have to use a framework or library in the application

Solution: Use Service to Worker to centralize control and request handling to retrieve a presentation model before turning control over to the view. The view generates a dynamic response based on the presentation model.

Service To Worker

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Service To Worker: Structure

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Service to Worker: Sequence Diagram

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Books

ALUR, Deepak et al. Core J2EE Patterns: Best Practices and Design Strategies, 2on Edition, Prentice Hall PTR, 2003.

FOWLER, Martin Patterns of Enterprise Application Architecture, Addison Wesley, 2002.

Webs

www.corej2eepatterns.com

java.sun.com/blueprints/corej2eepatterns/

java.sun.com/blueprints/guidelines/designing_enterprise_applications_2e/web-tier/web-tier5.html

References